The real issue isn't the exact time limit, it's the intent of the travel. If you move to another country with the intent to live there permanently and give up your US residence, you can theoretically lose GC status in one day. I can't imagine that actually happening so quickly, though.
As a practical matter, trips of less than 6 months are assumed to be temporary, and you're assumed not to have abandoned your status. It would be very rare to lose GC status with a trip of less than 6 months; some unusual evidence would have to come to the officer's attention that strongly suggests you didn't intend to keep your US residence.
After six months, the burden shifts around somewhat. You might be asked to demonstrate that you didn't intend to abandon your status. If you can't show any evidence that you intended to keep your status, there's a risk you may be assumed to have abandoned it.
After one year, a re-entry permit is required.
After two years, a returning resident visa is required.
Regardless of the length of the trip, be sure and file tax returns when they're due, and never declare yourself to be a "nonimmigrant" on your tax forms.
Also, be aware of the effect that trips of six months or more may have on naturalization, if you're thinking naturalization is in your future.
See the
Now that you're a permanent resident page from the USCIS for details, especially the "maintaining permanent residence" paragraph.
